The Upper Ziz basin located in the southeast of Morocco, has a total area of 4,351 km2. The surface water feeds El Hassan Addakhil dam, which insures water supply for the downstream cities of Errachidia, Rissani, Erfoud and others along the Ziz valley. This study aimed to evaluate the availability of water resources in this basin known by its arid climate and strong climatic changes. Several global hydrological models at different times were used to simulate the discharge at the outlet. The Statistical Downscaling Model (SDSM) method has been used to reduce the average rainfall and the temperature to predict future climate change related to various Representative Concentration Pathway (RCP) scenarios such as RCP 4.5 and RCP 8.5. The results of the hydrologic models are available, with an NSE of 0.8 for the monthly model during calibration and 0.77 at validation. Future precipitation shows an increasing trend in both scenarios. As for future mean temperature, it will recognize great seasonal variability, such as warming winter and spring and cooling summer and autumn. As a result, simulated future discharge will decrease by 26% under RCP 4.5 and by 24% under RCP 8.5 in the near future.